The pertinent prior art in this field is best illustrated by the flow diagrams, as presented, for example, in FIGS. 1 and 2 of the drawings, and those of the relevant literature, compare, in respect of FIG. 1; `Bulletin de la Societe Ceramique`, C 525, A 43, and in respect of FIG. 2 `Firma Eirich Fachberichte Ausgabe 21/1980`. It is evident therefrom that multistep and sophisticated processes are always involved.
Furthermore there are other known processes for the production of ceramic powders, which are characterised by spraying solutions or suspensions into directly or indirectly heated reaction chambers (e.g. spray roasters). Such processes have also often been described in the literature. A few examples originating from patent literature may be listed: U.S. Pat. No. 3,378,335, DE-AS 24 03 900 and European Patent 0 011 265.
Production of pre-sintered or reaction-sintered ceramic powders occurs in a known manner by means of plate push-type furnaces, chamber kilns, rotary kilns, pendulum kilns, as well as by using fluidised bed reactors and spray roaster furnaces.
The degree of pre-sintering and the chemical and physical properties of the ceramic powders produced by means of these processes are in such cases subject to considerable process-dependent fluctuations in the material properties, which may cause problems in subsequent process steps and yield inferior product qualities.
In the above-mentioned processes for the production of ceramic powders, in particular it is not possible for various reasons to provide the developing ceramic reaction product, especially the individual particles, which, for their part, create aggregates and agglomerates, with one and the same thermal and chemical treatment. This fact necessarily leads to the formation of particles exhibiting different chemical analysis and a widely scattered particle size distribution, which may not be corrected in a desired way by subsequent, and often undesired, milling procedures. Highly complex milling and upgrading processes have been discussed in this connection, such as, for example, in the work by Ochiai and Okutani in `Advances in Ceramics`, Vol 15, 16 (1984/85); Amer. Cer. Soc.
In directly heated pre-sintering furnaces (or reaction-sintering furnaces) e.g. in the spray roasting process, the quality of the furnace atmosphere is strongly dependent on the composition of the combustion gases and may not be freely selected in a desirable manner.
When producing, for example, alpha ferric oxide, which is an important raw material for ferrite production, the above-mentioned facts lead to the production of varying undesired contents of maghemite (gamma ferric oxide) and magnetite (ferrosoferric oxide), as has also already been described in literature. See Fick and Zenger `Advances in Ceramics` Vol. 17 and 18 (1984/85); Publisher: Amer. Cer. Soc.
Spray roasting is characterised by spraying droplets, which have a diameter of 20 up to 500 .mu.m into a roaster furnace; also have the above-mentioned references. Due to process conditions, the reaction and combustion gases form a cyclone, which causes the smaller droplets or agglomerates of the roasted products to experience a higher degree of uplift and receive therefore a different thermal treatment in comparison to larger droplets or larger agglomerates of the roasted products. This fact results in products being obtained consisting of primary single crystals having very different particle sizes. Furthermore, processes especially for presintering and reaction-sintering, which require an extended processing time, give rise to a crystal or particle growth to an extent resulting automatically in accumulation of originally finely dispersed impurities.
The undesired distribution of impurities and subsequent milling operations which, for economic reasons, are not sufficient to disperse impurities finely again are, together with the widely scattered particle size distribution, responsible for the formation of so-called ceramic duplex structures by utilisation of such ceramic powders in ready-sintered shaped ceramic parts which adversely affect the quality of the material properties.